This invention relates to a process for producing substantially fibre-free cellulose ethers which predominantly exhibit elastic properties, to corresponding cellulose ethers, to the use thereof as a superabsorbent material and as an adjuvant substance for achieving suitable rheological and water retention properties for the cosmetics, pharmaceutical and food sectors, and for industrial applications, e.g. as an additive for the sealing of cables (telecommunications cables, etc.).
Superabsorbers in the sense of this invention should be understood to be products which are capable, as powders or granular materials, of absorbing liquids (water, urine, wound secretions, blood, etc.) and of retaining liquids, even under stress at a pressure of 0.1 psi-1 psi, such as that which occurs when wearing plasters, nappies, bandages and articles of hygiene of all types, as well as denture fixative cream, for example.
According to the prior art, pulverulent synthetic, polyacrylate-based superabsorbers are used for hygiene products (e.g. bandages, etc.). Their superabsorbent properties amount to about 50 g liquid per g polymer. These products are not biodegradable, however. Taking into consideration merely the proportion of what are termed disposable nappies in the entire amount of domestic waste in Germany, which is currently about 2-3% thereof, it is understandable that possible means are being sought for the replacement of synthetic products by biodegradable or compostable substances, and for furnishing the latter with technical qualities, e.g. superabsorbent properties, which are at least equivalent.
Conventional uncrosslinked carboxymethyl celluloses (hereinafter also referred to as CMCs), which are digested in the presence of a caustic solution such as caustic soda, and which are etherified with an etherification agent, such as monochloroacetic acid for example, in a mixture an organic suspension medium and water, are conventionally considered to be products which are not superabsorbent (in this respect, see R. L. Whistler in xe2x80x9cIndustrial Gumsxe2x80x9d, page 704 et seq. (2nd Edition 1973)). These are usually uncrosslinked carboxymethyl celluloses which are washed with a mixture comprising suspension medium, which does not dissolve CMC, and water, and the fibrous structure of which can still clearly be identified under the microscope (see U.S. Pat. No. 2,715,124 in this respect). A carboxymethyl cellulose which is produced in this manner has different thickening properties depending on the cellulose raw material used and on the degree of polymerisation thereof (lignocellulose, cotton linters, crude linters, etc.), but generally exhibits no absorbent or superabsorbent properties. There has therefore been no lack of attempts to convert carboxymethyl cellulose, which is normally soluble in water, into an insoluble form and to improve the absorption properties by the use of crosslinking agents. Examples of crosslinking agents which have been described include 1,2-dichloroethane, epichlorohydrin, aldehydes such as formaldehyde, or metal salts which form complexes, such as chromium compounds for example (JP 04161431-A, J 63037143 A, U.S. Pat. No. 4,952,550, RD 349022 A). Moreover, there has been no lack of attempts to provide mixtures of modified carbohydrate polymers with synthetic polymers which swell in water, such as crosslinked polyacrylamides for example (EP 0131090, U.S. Pat. No. 4,021,257, U.S. Pat. No. 4,110,226, U.S. Pat. No. 3,574,188, EP 0056360, DE 3929400, DE 4328190 A1 and DE 4206857 A1). One particular disadvantage here, however, are the eco-toxicological aspects of the production, use and disposal of the crosslinked polymers. Thus special process technology measures for the protection of personnel and the environment are necessary when using polymers which are crosslinked with chloroorganic compounds or aldehydes. Moreover, the use of correspondingly crosslinked carboxymethyl celluloses in hygiene products for example (e.g. nappies, bandages) in which the CMC comes directly or indirectly into contact with the skin, can result in allergic reactions or in damage to the vegetative nervous system. Finally, the disposal of these materials can result in the contamination of groundwater due to leaching processes.
EP 0 201 895 B1 describes a process for producing a substantially non-fibrous, superabsorbent CMC, in which the superabsorbent product is obtained by dissolving the CMC in water and by the addition of a non-solvent (e.g. acetone or isopropanol).
However, the dissolution in water of a CMC or CMC cake which has already been produced, and the subsequent precipitation thereof by the addition of a non-solvent for the CMC, constitutes an additional process step which increases the cost of the process.
The object of the present invention was therefore to provide cellulose ethers, particularly a carboxymethyl cellulose, which exhibits improved absorbent properties, particularly superabsorbent properties, without the use of toxic or environmentally harmful substances. As regards the process technology employed, the object was for the product to be capable of being produced in a simple, inexpensive manner.
Surprisingly, it has been shown that simply by changing the conditions of alkalification during the production of corresponding prior art cellulose ethers, these products can be improved from a technical and eco-toxicological point of view. It has been shown that in aqueous solution, even without crosslinking reagents, the products described in the present application form high-strength gels which exhibit improved water retention. When these products are used as powders, e.g. in adhesive plasters, nappies, bandages, denture fixative creams, etc., a considerably improved water retention is observed in relation to liquids such as blood, wound secretions, urine, etc. Because toxic substances are used neither in the product itself nor in the process technology employed for the production thereof, it is possible to use these substances harmlessly for food, cosmetics and pharmaceutical applications, in addition to their industrial applications. Moreover, compared with the prior art, the water-soluble cellulose ethers which are produced according to the invention exhibit different rheological properties, particularly elastic properties, due to which it is possible to differentiate them from conventional cellulose ethers, particularly CMCs. The consequence of this is that the products which are claimed according to this inventionxe2x80x94on their own or in combination with additional adjuvant substancesxe2x80x94can even be used in areas in which conventional cellulose ethers exhibit deficiencies, e.g. due to shortcomings in their limits of flow (e.g. in the construction of underground curtain walls and uncased concrete piles, etc., or in coating materials (dispersion or silicate paints, etc.)).